1. Field of the Invention
The present invention relates to a device for scanning the surface of a sample covered with a liquid. Such devices are disclosed, for example, in Lambelet, P., M. Pfeffer, A. Sayah and F. Marquis-Weible (1998), “Reduction of tip-sample interaction forces for scanning near-field optical microscopy in a liquid environment”. Ultramicroscopy 71(1-4): 117-121; Nitz, H., J. Kamp and H. Fuchs (1998). “A combined scanning ion-conductance and shear-force microscope”. Probe Microsc. 1: 187-200; and Schäffer, T. E., B. Anczykowski and H. Fuchs (2006), Scanning Ion Conductance Microscopy, Applied Scanning Probe Methods, B. Bhushan and H. Fuchs. Berlin, Heidelberg, N.Y., Springer Verlag. 2: 91-119.
2. Description of the Background Art
A known device 10 according to the preamble of claim 1 is shown schematically in FIG. 1. The device comprises a probe which is formed by a pipette 12 which tapers to a fine tip 14 at its lower end in the diagram in FIG. 1. The pipette 12 is furthermore in contact with a piezo-element 16 by which the tip 14 of the pipette 12 can be set in vibration by.
FIG. 1 also shows a sample container 18 in which a sample 20 shown schematically is located. The sample container 18 is filled with a liquid 22 which completely covers the sample 20. The sample 20 could, for example, comprise living cells which can only exist in liquid. Another reason for covering the sample 20 with a liquid, more accurately with an electrolyte, resides in the possibility of carrying out ion conductivity measurements which will be described in detail below.
The sample container 18 is located on an XYZ scanner 24 so that the sample container with the sample 20 can be moved relative to the pipette 12. Due to the relative movement, the surface of the sample 20 can be scanned with the tip 14 of the pipette 12.
The device 10 from FIG. 1 further comprises a laser 26 whose beam 28 is focussed onto the tip 14 in the quiescent state by a focussing device not shown. The laser 26 and the pipette 12 are positionally fixed with respect to one another, for example, by mounting both on the same experimental table. Thus, the distance between the laser 26 and the tip 14 of the pipette 12 does not change during scanning so that the laser beam should always be focussed onto this tip during the relative movement between the sample and the pipette tip 14.
Finally, there is provided a detector 30 which receives the laser beam 28 after this has been reflected at the tip 14. The reflected laser light 28 is modulated by the vibration of the tip 14 produced by the piezo-element 16. By means of these modulations, the vibrations of the pipette tip 14 can be detected with the aid of the detector 30. In the present document, the interaction of the laser beam with the tip 14 is generally designated as “scattering”. The term “scattering” in particular embraces reflection from the tip and transmission which is obtained, for example, when the tip vibrates out from the light path of the laser beam.
When the pipette tip 14 is brought very close to the surface of the sample 20, shear forces occur which influence, for example, damp, the amplitude, phase and/or frequency of the vibration of the tip 14. The damping of the vibration is in turn detected with the aid of the detector 30. As a result, the distance between the tip 14 and the sample 20 can be determined. For example, the XYZ scanner 24 can be driven in such a manner that the damping of the oscillation and therefore the distance between the tip 14 and the sample can be kept constant when scanning the sample 20. The movements of the XYZ scanner carried out when scanning the surface of the sample 20 can be recorded by a computer (not shown) and a topographical picture of the surface can be generated from these. The device of FIG. 1 is therefore designated as a shear force microscope.
However, in the known device of FIG. 1, problems with the reliability of the signals frequently arise. It appears to be difficult to keep the laser beam 28 focussed on the tip 14 of the pipette when this is surrounded by the liquid 22. In order to avoid these problems, attempts have been made to keep the level of the liquid 22 sufficiently low that an upper section of the tip 14 projects above the liquid level and to focus the laser beam 28 onto this upper section without it needing to pass through the liquid. However, this has proved to be difficult in practice since a sufficiently low liquid level above the sample can only be produced with difficulty and can be maintained only with difficulty during the investigation because a part of the liquid is continuously evaporating. Even if the sample can be kept continuously covered with liquid, the following problems arise: if the liquid level is kept low and is thereby present on a section on which the tip vibrates with comparatively large amplitude, the optical signals obtained are perceptibly modified by small changes in the liquid level as a result of evaporation and thereby falsified. If a higher liquid level is used, which is present on a section of the tip on which the tip vibrates only with comparatively low amplitude, the signals are certainly more stable but in return significantly weaker.
In order to avoid these problems a “diving bell” structure is used in Koopman, M., B. I. de Bakker, M. F. Garcia-Parajo and N. F. van Hulst (2003) “Shear force imaging of soft samples in liquid using a diving bell concept”. Appl. Phys. Lett. 83(24): 5083-85 but this is comparatively complex.
It is the object of the present invention to improve a device of the type specified above in such a manner that it allows reliable focussing of the light onto the tip of the probe.
This object is achieved in the device of the type specified above whereby a boundary surface at which the light enters the liquid is located on the path of the light between the light source and the tip of the probe, wherein the boundary surface is positionally fixed with respect to the probe.